Washington — A third-party presidential candidate is likely in 2012, introducing considerable uncertainty into the battle to win control of Congress, Democratic pollster Stanley Greenberg says.

“There is going to be a third-party candidate,” Mr. Greenberg said Friday at a Monitor-hosted breakfast for reporters. Greenberg, who served as President Bill Clinton’s pollster, also has worked with international clients including Nelson Mandella and Tony Blair. He is co-founder, along with James Carville, of Democracy Corps, which provides polling and strategic advice to progressive candidates and groups.

Greenberg offered several reasons for thinking a credible third-party presidential candidate will emerge. A key reason is voters’ sour mood. An NBC News/Wall Street Journal poll released earlier this week found that just 22 percent of the public thinks the nation is heading in the right direction. When Ross Perot ran as an independent presidential candidate in 1992, “those were happy times compared to now in terms of the mood of the country,” Greenberg said.

Undecided voters also are likely to favor a third-party candidate, Greenberg argued. Undecideds “are more Democratic than Republican,” he said. “They hate both parties equally. They are marginally unfavorable to Obama but they hate Romney.... I don’t know if they will be brought back to Obama. I think more likely they will vote for some third-party, anti-politics candidate that will be out there.” He noted that Bill McInturff, who runs the NPR poll with Greenberg, thinks the third-party vote could go as high as 20-25 percent.

The likely presence of a third-party candidate means “there is enormous uncertainty” in terms of the battle to control Congress since it could affect the coattail effect of the top of the ticket. In the House, Republicans picked up 63 seats in 2010. Democrats need to have a 25 seat net gain to regain control in 2012. Non-partisan political analyst Charlie Cook, editor of the Cook Political Report, wrote recently that the “most likely outcome appears to be a Democratic gain of five to ten seats.”

A third-party candidate is more likely to pull support from the Republican presidential candidate, offering an alternative to those who oppose Obama, Greenberg said. “I do think it strongly favors the president,” Greenberg said.

Perhaps not surprisingly, Greenberg was dismissive of the Republican Party’s prospects, noting it had failed to pick up a significant number of voters despite a landslide 2010 victory in congressional races and a bleak economic picture. “The Republican Party is in trouble,” he said. “There are no more people calling themselves Republican through this whole process even though they had a landslide election in 2010…. It has become a cult. Independents are now equal to it,” in relative political strength.